1. Field of the Invention
This invention relates to methods for making rubber reinforced copolymers, and more particularly relates to methods for making rubber modified graft copolymers of monovinylidene aromatic monomers and unsaturated nitrile monomers grafted on a rubbery substrate by mass polymerization.
2. Description of the Related Art
Rubber modified graft copolymers of a monovinylidene aromatic such as styrene and an unsaturated nitrile such as acrylonitrile having particulates of rubber, generally an alkadiene rubber, dispersed throughout a copolymeric matrix (conventionally referred to as ABS resins) are employed in a wide variety of commercial applications such as packaging, refrigerator linings, automotive parts, furniture, domestic appliances and toys. It is well known that the physical properties of an ABS resin such as toughness (i.e., the combination of elongation and impact strength), at both room and lower temperatures, are affected by the grafted styrene-acrylonitrile copolymers of the rubber substrates and by the size, composition and morphology of the dispersed rubber particles and/or the concentration of the rubber substrates in the rubber-reinforced copolymers. For example, to achieve the balance of physical properties required in many applications, the rubber particles are necessarily dispersed through the copolymer matrix at a relative size of typically 0.5 microns and 5 microns, typically yielding a low gloss product as a result of the rubber sizes being at least 0.4 microns as the average particle size, more typically greater than 0.5 microns.
There are two well known manufacturing processes among many different bulk (mass) ABS processes. The first one is a multi-zone, continuous plug flow process. The second is a bulk/suspension process. The multi-zone plug flow bulk HIPS/ABS process was described in early U.S. Pat. Nos. 2,646,418; 2,694,692; 2,727,884 and 3,243,481 and in many other patents that followed, such as U.S. Pat. Nos. 4,874,815; 4,785,051; 4,713,420; 4,640,959; 4,612,348; 4,387,179; 4,315,083; 4,254,236; 4,417,030; 4,277,574; 4,252,911; 4,239,863; 4,221,883; 4,187,260; 3,660,535; 3,243,481--all of which are incorporated herein by reference.
Multizone plug flow bulk processes include a series of polymerization vessels (or towers), consecutively connected to each other, providing multiple reaction zones. Butadiene (BD) rubber (stereospecific) is dissolved in styrene (ST) or in styrene/acrylonitrile (ST/AN), and the rubber solution is then fed into the reaction system. The polymerization can be thermally or chemically initiated, and viscosity of the reaction mixture will gradually increase. During the reaction course, the rubber will be grafted with ST/AN polymer (grafted SAN) and, in the rubber solution, bulk SAN (referred to also as free SAN or matrix SAN or non-grafted SAN) is also being formed. At a point where the free SAN (i.e. non-grafted SAN) can not be "held" in one single, continuous "phase" of rubber solution, it begins to form domains of SAN phase. The polymerization mixture now is a two-phase system. As polymerization proceeds, more and more free SAN is formed, and the rubber phase starts to disperse itself as particles (rubber domains) in the matrix of the ever-growing free SAN. Eventually, the free SAN becomes a continuous phase. This is actually a formation of an oil-in-oil emulsion system. Some matrix SAN is occluded inside the rubber particles as well. This stage is usually given a name of phase inversion. Pre-phase inversion means that the rubber is a continuous phase and that no rubber particles are formed, and post phase inversion means that substantially all of the rubber phase has converted to rubber particles and there is a continuous SAN phase. Following the phase inversion, more matrix SAN (free SAN) is formed and, possibly, the rubber particles gain more grafted SAN. When a desirable monomer conversion level and a matrix SAN of desired molecular weight distribution is obtained, the reaction mixture is "cooked" at a higher temperature than that of previous polymerization. Finally, bulk ABS pellets are obtained from a pelletizer, after devolatilization where volatile residuals are removed.
For the mass/suspension process, U.S. Pat. Nos. 3,509,237; 4,141,933; 4,212,789; 4,298,716, describe those processes. A monomer solution of rubber substrate is charged to a reactor, and polymerization is carried out to reach a given solids level where phase inversion occurs. After phase inversion, the polymerization mixture is transferred to a reactor and mixed with water/suspending agent/surface-active agent. Polymerization is then completed in this suspension system.
Furthermore, U.S. Pat. No. 3,511,895 describes a continuous bulk ABS process that provides controllable molecular weight distribution and microgel particle size using a "three-stage" reactor system, for extrusion grade ABS polymers. In the first reactor, the rubber solution is charged into the reaction mixture under high agitation to precipitate discrete rubber particle uniformly throughout the reactor mass before appreciable cross-linking can occur. Solids levels of the first, the second, and the third reactor are carefully controlled so that molecular weights fall into a desirable range.
Continuous mass polymerization processes employing continuous-stirred tank reactors have been employed in the production of high impact modified polystyrene wherein for example a process involving three reaction steps wherein the first step is a continuous-stirred tank reactor, the second step is a continuous-stirred tank reactor, and the third step is a plug-flow reactor.
In such a system, the first continuous-stirred tank reactor would be charged with styrene monomer having polybutadiene polymer dissolved therein, wherein the styrene monomer and polybutadiene polymer would be reacted sufficiently until phase inversion, at which point discrete particles of rubbery phase would separate from a second phase of polystyrene and styrene monomer, this phase inverted product would then be charged to a second continuous-stirred tank reactor wherein further monomer conversion takes place, followed by reaction of product from the second continuous-stirred tank reactor in a plug-flow type reactor to obtain final conversion. There is a desire to employ continuous-stirred tank reactors, due to their superior ability to control temperature and heat transfer in the reactor, in the mass polymerization of ABS type graft copolymers. However, applicant has discovered that using a first step involving a continuous-stirred tank reactor wherein phase inversion occurs does not yield uniform grafting of the rubber and results in an undesired precipitation of rubber particles before high levels of grafting onto the rubber are achieved. Inadequate grafting leads to poor product performance including reduced levels of impact strength.
Furthermore, different processes of ABS manufacturing give different properties to the final ABS products. One of these properties is the surface gloss of the end products, and technology development to produce ABS materials that could meet with different gloss requirements is still an on-going task for the ABS industry.
The gloss of an ABS product is partially the result of molding conditions under which the product is manufactured. However, for a given molding condition, the rubber particle size (diameter) of the ABS material is a major contributing factor to the gloss. In general but not always, ABS materials from emulsion processes produce rubber particles of small sizes (from about 0.05 to about 0.3 microns). Therefore, high gloss products are often made from emulsion ABS materials. On the other hand, ABS materials from mass processes usually form rubber particles of large sizes (from about 0.5 to 5 microns). Therefore, low gloss products are often made using the bulk ABS materials. Although it is possible to produce small particles using bulk processes, the gloss and impact resistance balance will be difficult to reach.
In order to combine advantages offered by emulsion and bulk ABS materials for better gloss and impact balances, these two type of ABS materials are blended at different ratios to obtain bimodal particle size distributions. For example, U.S. Pat. Nos. 3,509,237 and 4,713,420 presented technology of this type so that high surface gloss and good impact strength were achieved.
Also, ABS materials of bimodal particle size distributions that gave gloss readings from 80 to 99 percent were directly made from a bulk process, described in U.S. Pat. No. 4,254,236. To make this type of bulk ABS, two feed streams were simultaneously charged to the reaction system. One of the feed streams was a mixture containing rubber substrate, monomers and a superstrate (matrix polymer) of the monomers. The other was a monomer solution of the rubber substrate. Another U.S. Pat. No. 3,511,895 described a bulk process where rubber particles are formed by dispersing and precipitating polymeric butadiene rubber as discrete droplets in the reaction mixture, leading to bulk ABS of high gloss. With such process conditions, the desirable "cell" morphology of rubber particles could hardly be obtained, resulting in low impact strength. Another U.S. Pat. No. 4,421,895 described a continuous process for relatively small sizes (averages were 0.5 to 0.7 microns) of rubber particles for bulk ABS. However, those average particle sizes are not uncommon for bulk ABS materials and are still not small enough to contribute to the high gloss performance. However, efforts have been made to produce smaller sizes of rubber particles in bulk processes using a particle disperser after phase inversion, described in EP 0 376 232 A2. The average particle sizes were able to be reduced to a volume average diameter of 0.4 microns. But, the respective gloss value was 89%, that was only at the high end of the regular bulk ABS "reduced/lower" gloss range.
Overall, current technology described above has not been able to produce, by a bulk process alone, ABS materials of rubber particles of "cell" morphology with monomodal particle size distributions and with average particle sizes less than 0.3 microns of number average diameter, without compromising impact resistance properties.
To synthesize ABS polymers with high performance by bulk processes, three aspects are essential among many others. These three aspects are grafting of the rubber substrate prior to phase inversion, particle formation during phase inversion, and cross-linking of the rubber particle at the completion point of the bulk ABS polymerization. However, the above mentioned bulk ABS processes are somehow deficient by different degrees in controlling and in adjusting the grafting, the phase inversion, and the crosslinking. Accordingly, there is a desire to provide a continuous mass polymerization process which yields the desired rubber morphology and maximizes grafting thereby allowing a minimization of rubber use for a given level of property performance. Additionally, there is a desire to provide a bulk process capable for producing ABS resins of low gloss as well as high gloss.